In drying a sheet of wood veneer, a so-called contact or press type dryer is known. In this dryer, a veneer sheet is dried by a plate which is heated to a desired level by any suitable means such as steam, heated oil, electrical heater, etc. and brought into direct contact engagement with the veneer sheet to be dried, as disclosed by Publication of Examined Japanese Patent Application No. 52-24085 (1977) for "Method of drying veneer", or by a similarly heated plate which is brought into contact with an intermittently movable steel belt used as means for intermittently conveying the veneer sheet through the dryer and also for allowing the heat from the plate to conduct therethrough to the veneer sheet when the belt is at a stop during its intermittent movement and while it is then placed in contact with the sheet, as disclosed by Publication of Examined Japanese Utility Model Application No. 59-34872 (1984) for "Continuous press dryer for sheet material such as thick veneer". Besides the above two veneer dryers, various changes and modifications therefrom have been disclosed by similar publications.
As compared with a so-called hot air type in which the veneer is dried by heated air enforced to circulate in a conditioning chamber or a dry kiln, the above contact type dryer is known to offer an advantage in thermal efficiency in drying operation. The heating surface of the plate that is to be in engagement directly with the veneer sheet or indirectly via the conveying belt has been usually formed so as to present a flat surface when the plate is placed in its inoperative or non-engagement position away from the veneer sheet.
With a heating plate having one surface on either side thereof engageable with the veneer directly or indirectly, however, a problem will be encountered with, as will be described below, if the engageable surface of the heating plate is formed so as to present a flat surface in its inoperative condition.
When the heating plate is brought into a contact engagement with an incoming green veneer sheet containing a noticeable amount of moisture and hence relatively cold, the heating surface of the plate is cooled down rapidly by the sheet and consequently contraction stress is produced in the plate adjacent its heating surface in engagement with the cold veneer sheet. Though no fear of the problem will occur if such stress takes place on both opposite surfaces of the plate because of their substantially simultaneous contact engagement with green veneer sheets so that the stresses on the opposite surfaces balance, and therefore offset, each other, if the dryer is so arranged that one surface on either side of the heating plate is engaged directly or indirectly with the cold veneer sheet, that surface will be deformed into a "cup" shape, or curved into a concave form, under the influence of the contraction stress. Consequently, not only the contact of the heating plate with the veneer sheet becomes uneven, so that the efficiency in heat conduction to the veneer sheet is decreased during the critical early period of drying, but also the steam emitted from the wet veneer sheet by drying thereof tends to be confined in a small space formed by the concave-shaped surface, thereby delaying the diffusion of the steam and thus affecting the efficiency in drying. As a result, the veneer sheet is caused to be contracted earlier at its edges than other portions so that a trouble such as cracks or checks may be produced in the veneer sheet in progress.
To prevent the heating plate from being deformed by the contraction stress, the plate may be constructed with an extremely high magnitude of geometrical moment of inertia so as to increase its rigidity to such an extent that can resist the above contraction stress, e.g. by increasing its thickness or providing reinforcement ribs. However, provision of such heating plate will inevitably increase its weight and therefore the manufacturing cost thereof, but also it will make the dryer itself larger in size because of the necessity to construct the plate lifting mechanism strong enough to resist the heavy weight of the plate and also to arrange the veneer conveyer long and large-sized. In addition to such inconveniences, the thermal efficiency of the plate will be reduced because the area of its heat radiation surfaces will be increased.
Alternatively, the heating plate may be pressed directly or indirectly against the veneer sheet until its heating engagement surface becomes substantially flat. However, such pressing by the plate will excessively compress the veneer sheet to thereby reduce its thickness. Therefore, this method, if used, would be limited to drying veneer of special wood material.